Adding note about clearing stubs between tests.

While testing an application I ran into some odd behavior where stubs were not being cleared between tests automatically. Additionally if I redefined the same stub that had already been defined I'd get the old behavior. I wanted to provide an addition to the documentation so it was clear for others so they didn't fall into the same trap as I did.

Demo of the behavior
https://gist.github.com/pcorliss/7832781